In the power steering systems of automobiles, solid particles developed through wear of the system parts are present in the fluid causing contamination, increased friction and raised fluid temperatures. These particles also increase deterioration of the hoses and seals. Therefore, it is desirable to refine the power steering fluid by filtering such particles. Presently, there are known filters which employ a paper filtering element together with a relief valve made of a combination spring, valve disc and seat arrangement. Such known filter devices are generally complex, involving the spring and valve seat arrangement, and therefore are expensive to manufacture. Also, such known filter devices are often not reliable due to the spring arrangement employed. There are other known types of filter devices being used in automobiles which are less expensive than the above-noted power steering filter devices but which are not suitable for the same purpose of filtering the fine particles existent in power steering systems and, consequently, such known filters also do not provide adequate and reliable relief valve means in the filter. Examples of such filters devices are the floating oil screens used in the oil sumps of engines as described in U.S. Pat. No. 2,192,432 to Gulick, U.S. Pat. No. 2,508,952 to Kline and U.S. Pat. No. 3,662,887 to Uhlhorn, Jr. Such patents disclose devices mounted or floating in the oil sump of an internal combustion engine to prevent sludge and the like from entering the oil pump. When the screen becomes badly clogged with foreign matter, the suction pull on the screen will open a valve formed by a portion of the screening element and a wall portion to allow oil flow without screening. In such known oil sludge filter devices, the wire screen element is intended for filtering out relatively large particles and sludge and is inappropriate for filtering fine particles, even if the screened mesh openings were made smaller. The screen material is highly flexible and subject to random deformation and, therefore, the screen valve may easily open under unintended variable and minor conditions not due to clogging, such as slight variations in suction pressure and very small build ups of heavy sludge against the very flexible screen causing a suction sufficient to move the screen valve into an open position. Also, the non-rigid nature of the screen material affords it no spring force to return it to its original closed position once the minor suction fluctuation is reduced to normal. This absence of rigidity, or memory, could result in the screen valve opening and remaining open even after the minor suction fluctuation is removed. This random deformation and uncontrolled shifting of the position of the screen element in such oil sludge filters results in a relief valve which cannot be designed for opening under predetermined, controlled conditions of filter clogging. Also, the valve seal formed by an element of the screen with the wall is generally a loose-acting seal made under slight pressures of the screen element on a wall. Thus, such a loose and uncontrolled sealing means as used in these oil sludge filter devices is not suitable for the requirements in filtering the fine particles in a power steering system.